Fix flakes in testMultipleProbes

If a conclusive probe finishes before other probes start, some probes may
not call Network#openConnection, as the executor is shut down
immediately.

Test: atest NetworkMonitorTest#testMultipleProbesOnValidNetwork \
          --rerun-until-failure 500
Bug: 139314310
Original-Change: https://android-review.googlesource.com/1299380
Merged-In: Ica21c23af0472edc6ead123365b39e42e69f85cb
Change-Id: Ica21c23af0472edc6ead123365b39e42e69f85cb